The present invention relates to rotary fluid pressure devices, such as low-speed, high-torque (LSHT) gerotor motors, and more particularly, to an improved high pressure shaft seal assembly for use in such devices.
Gerotor motors of the LSHT type are normally classified, in regard to their valving configuration, as being of either the “spool valve” or “disc valve” type. As used herein, the term “spool valve” refers to a generally cylindrical valve member in which the valving action occurs between the cylindrical outer surface of the spool valve and the adjacent internal cylindrical surface (bore) of the surrounding housing. By way of contrast, the term “disc valve” refers to a valve member which is generally disc-shaped, and the valving action occurs between a transverse surface (perpendicular to the axis of rotation) of the disc valve and an adjacent transverse surface of the housing (stationary valve surface). Furthermore, among disc valve motors, there is also a sub-category which may be referred to as “valve-in-star” motors, in which the gerotor star member itself has a disc valve integral therewith, an example of such a motor being illustrated and described in U.S. Pat. No. 4,741,681, assigned to the assignee of the present invention, and incorporated herein by reference.
Although the present invention may be utilized with LSHT gerotor motors having any one of a number of different valving configurations, it is especially suited for use with spool valve motors, and will be described in connection therewith. It should be noted that the use of spool valve gerotor motors has typically been limited to relatively smaller motors, having relatively lower flow and pressure ratings. This has been true partly because of certain inherent limitations in spool valve motors, resulting from the radial clearance between the spool valve and the adjacent cylindrical surface (“stationary valve surface”) of the housing. This radial clearance provides a potential cross port leakage path such that, as the radial dimension of the clearance increases, the volumetric efficiency (and overall efficiency) of the motor decreases.
One of the problems associated with spool valve type gerotor motors is that, as customers seek to continually increase the torque output of the motor by increasing inlet pressure, there is a tendency for the spool valve to “collapse” under the higher pressure, thus increasing the radial clearance between the spool valve surface and the stationary valve surface of the housing. As noted previously, the increasing radial clearance results in decreasing volumetric efficiency of the motor, which is always undesirable from the viewpoint of the customer.
It has been known to those skilled in the art that one possible solution to the problem of a “collapsing” spool valve is to increase the “case drain” pressure, i.e., the pressure in a chamber disposed within the interior of the motor, including the volume disposed within the hollow cylindrical spool valve. The typical way of increasing case drain pressure is simply to restrict flow out of the case drain port, thereby causing a buildup in pressure within the case drain region. Therefore, instead of the case drain region pressure being at reservoir pressure, the case drain region pressure may be elevated to somewhere in the range of 1,000 psi to 2,000 psi., with that pressure opposing the tendency of the spool valve to collapse. As is well known to those skilled in the art, if flow out of the case drain region is restricted, the pressure in the case drain region will typically be about mid-way (or slightly greater) between the inlet pressure and the outlet pressure.
Unfortunately, increasing case drain pressure has not been considered an acceptable solution to the collapsing spool problem because the shaft seal assembly (i.e., the seal between the housing and the rotating output shaft) becomes worn much faster than would otherwise be the case, thus necessitating much more frequent downtime of the motor for replacement of the shaft seal assembly. More frequent replacement of motor shaft seals, and the associated downtime for the motor, is also not acceptable from the viewpoint of the customer.